Photography is much used in fabrication of microstructures in various electronic devices such as semiconductor devices, liquid-crystal devices. Recently, the increase in the technical level of large-scale integration and microfabrication is great, and it is desired to further improve the technique of photoresist micropatterning in photolithography.
At present, for example, photolithography in the forefront of the region of high-technology has made it possible to form a photoresist micropattern having a line width of 90 nm or so, and further studies and developments are being made for micropatterning to a higher level to a line width of 65 nm or so.
For attaining micropatterning to such a higher level, in general, some methods of improving photoexposure devices or photoresist materials may be taken into consideration. Regarding the method of improving photoexposure devices, there may be mentioned a method of employing short-wave light sources of F2 excimer laser, EUV (extreme-ultraviolet ray), electron ray, X ray, soft-X ray, and a method of employing lenses having an increased numerical aperture (NA).
However, the method of employing such short-wave light sources requires an additional expensive photoexposure unit. On the other hand, the method of employing such increased-NA lenses is problematic in that, since the resolution and the focal depth range are in a trade-off relationship, the increase in the resolution may lower the focal depth range.
Recently, liquid immersion lithography has been reported as a technique of photolithography capable of solving these problems (for example, see Non-Patent References 1 to 3). This method is for photoresist patterning through photoexposure of a photoresist film formed on a substrate, in which, in the photoexposure light pathway between the photoexposure device (lens) and the photoresist film, a liquid for liquid immersion lithography having a predetermined thickness is made to be on at least the photoresist film, and the photoresist film is exposed to light in that condition to thereby form a photoresist pattern. In the method of liquid immersion lithography, the photoexposure light pathway space, which is an inert gas such as air or nitrogen in conventional methods, is substituted with a liquid for liquid immersion lithography having a larger refractive index than that of the space (vapor) and having a smaller refractive index (n) than that of the photoresist film (for example, pure water, fluorine-containing inert liquid), and the advantage of the method is that, even though a photoexposure light source having the same wavelength level as that in conventional methods is used therein, the method may attain a high-level resolution like the case that uses a photoexposure light having a shorter wavelength or uses a high-NA lens and, in addition, the method does not result in the reduction in the focal depth range.
To that effect, the process of liquid immersion lithography is much noticed in the art, since it realizes photoresist patterning of high resolution to a good focal depth at low costs, using any lens actually mounted on the existing photoexposure devices therein.
However, in the process of liquid immersion lithography, the photoexposure is attained while a medium for liquid immersion lithography is made to be between the lens for photoexposure and the photoresist film, and therefore the method is problematic in that the photoexposure device may be damaged by the component released from the photoresist film into the medium for liquid immersion lithography (for example, the crystal material for the lens for photoexposure may be fogged, and as triggered by it, the transmittance of the lens may be lowered and the exposure may be uneven).
To solve the problem, employed are a method of improving the photoresist material to thereby prevent it from releasing the constitutive component, and a method of providing a layer of a protective film on the photoresist layer to thereby prevent the photoresist from releasing the constitutive component. However, the former method is limited in its development in point of the photoresist material and has another problem in that it could be hardly applicable to a wide variety of photoresists; and even the latter method could not completely prevent the photoresist from releasing the constitutive component.
Given that situation, another method of solving the above-mentioned problem has been proposed, still using the photoresist and the protective film now widely used in the art. The method comprises cleaning the optical lens member of the photoexposure device that is kept in contact with the medium for liquid immersion lithography, using a cleaning liquid (for example, see Patent Reference 1).
However, the cleaning liquid described in the patent publication comprises an organic, ketone-type or alcohol-type solvent; and even when said cleaning liquid is used, the effect of the method of removing the component released from photoresist, which is a high-risk factor of contamination, is insufficient, and the method could hardly keep the optical properties of photoexposure devices.
Other cleaning methods comprising cleaning the optical lens member of the photoexposure device that is kept in contact with the medium for liquid immersion lithography, using cleaning liquids, have been proposed (for example, see Patent Reference 2).
Non-Patent Reference 1: “Journal of Vacuum Science & Technology B”, USA, 1999, Vol. 17, No. 6, pp. 3306-3309
Non-Patent Reference 2: “Journal of Vacuum Science & Technology B”, USA, 2001, Vol. 19, No. 6, pp. 2353-2356
Non-Patent Reference 3: “Proceedings of SPIE”, USA, 2002, Vol. 4691, pp. 459-465
Patent Reference 1: JP-2005-157259A
Patent Reference 2: US 2005/205108 A1